Light emitting devices may generate light in response to an applied current or voltage. A light emitting device may include a light generation source. The light generation source may be, for example, a filament, used in incandescent light emitting devices, or an emission layer (abbreviated as EML hereinafter), used in organic light emitting diodes (OLED). Only a fraction of the light generated by the light generation source of a light emitting device may leave the light emitting device. The remaining fraction of the light generated by the light emitting device that does not leave the light emitting device may be converted to heat or otherwise not contribute to the quantity of light emitted by the light emitting device.
For a multi-layered light emitting device, such as light emitting diodes including inorganic light emitting diodes (LED) and organic light emitting diodes (OLED), a number of mechanisms may contribute to the remaining fraction of light that does not leave the light emitting device. The mechanisms may trap a majority of the light emitted by an EML within the multi-layered light emitting device. Due to the layered structure of a multi-layered light emitting device, light emitted by the EML within the light emitting device may become trapped within one or more layers of the light emitting device, e.g., due to waveguide modes, or absorbed by a metal electrode, e.g., by a plasmon mode.
For example, a conventional OLED may include several organic material layers sandwiched between an anode and a cathode. The OLED may only emit approximately 20% of the light emitted by the EML. The rest of the light emitted by the EML is either trapped inside one of the organic layers (commonly referred to as a waveguide mode), trapped inside a substrate (commonly referred to as a substrate mode), or absorbed by the cathode (commonly referred to as a plasmon mode).
Among the aforementioned trapping or loss mechanisms, the plasmon mode (sometimes referred to as plasmonic loss) may absorb approximately 40% of the light emitted by the EML and thus, substantially reduces the quantity of light emitted by the light emitting device. Reducing the plasmonic loss of a multilayer light emitting device may increase the quantity of light emitted by the multilayer light emitting device and improve the efficiency of the multilayer light emitting device.
Multilayer light emitting devices are conventionally categorized as either top emitting or bottom emitting. Multilayer light emitting devices are categorized as bottom emitting devices if light emitted by the EML is emitted out of the light emitting device by passing through a transparent or semi-transparent bottom electrode that is disposed on a substrate on which the light emitting device was fabricated. Multilayer light emitting devices are categorized as top emitting devices if light emitted by the EML is emitted out of the light emitting device by passing through a lid added to the light emitting device after fabrication.
Plasmon loss in light emitting devices may be reduced by replacing the regular bulk (or thick) metal electrode with a thin metal film. However, existing thin metal film electrode structures, as known in the art, are only directly applicable to top emitting devices. Thin metal film electrode structures, as known in the art, for bottom emitting structures require a mirror layer on a top side of the structure.